3hp Briggs will run without air filter but carb covered but wont run with air filter

Hello



I have a 3hp Brigs and Stratton engine on my rotavator/tiller and it has trouble running. It seems like it wants to start but it will not keep a steady tick over as it keeps cutting out after a few seconds. We took the air filter off the carb and I covered the carb with a gloved hand and to my amazement it started and ran quite smoothly. So I left it running to warm the engine whilst my hand was covering the carb after a few minutes I removed my hand and the engine died. I tried to start it again but it would not fire or start with the air filter in place or removed so I put my hand over the carb with the filter top removed and it started fine and ran ok until I removed my hand.



The question is what could be causing this strange problem? We have never encountered this before and although it has always been a problem to start the engine with a little prime with fuel it has normally started then ticked over without any problems.



Thanks in advance.